Purpose: To reproducibly generate fresh preparations of virus for independent experiments.Summary: Fresh virus sample for an experiment is generated by a primary infection of exponentially growing host cells from a master stock of virus. The infected host is allowed to lyse until the culture is cleared. The lysate is filtered to remove any large cellular debris, then the viral-size fraction is concentrated from the filtered lysate and washed with buffer using a centrifugal concentrator. The viral concentrate is stored at 4°C protected from light and should be used for an experiment within 1-2 days. On the day of (or the day before) the experiment, an MPN assay should be set up to assess infectivity of the fresh viral concentrate.